climate engineering a review of aerosol approaches to changing the global energy balance

CLIMATE ENGINEERING

A REVIEW OF AEROSOL APPROACHES TO CHANGING THE

GLOBAL ENERGY BALANCE

ROBERT E. DICKINSON Institute of Atmospheric Physics, University of Arizona, Tucson, AZ 85721, U.S.A.

Abstract. As global greenhouse warming continues to intensify, it is likely that demands to employ technologies of climate engineering will become increasingly insistent. This paper addresses the possibility of 'canceling' the radiative effects of the increasing greenhouse gases through solar reflectors. Two promising approaches, according to COSEPUP (1992), are the employment of aerosols in the stratosphere, directly as reflectors, or in the troposphere, for the 'seeding' of clouds to increase cloud amounts and brightness. Besides technological and economic feasibility, such schemes could be relatively reversible, and describing their impact may be within the reach of future scientific study.

The climate system is not yet sufficiently understood for such actions to be warranted. However, there is considerable potential for an increased understanding of what such actions might do through the study of the role of similar aerosols already added to the climate system. In particular, the most intense volcanoes (e.g. Pinatubo) supply the stratosphere with enough aerosol over a period of a year or two to cancel out greenhouse warming from a resulting doubling of carbon dioxide. Furthermore, the addition of sulfate aerosols to the troposphere from the burning of fossil fuel may already be canceling out globally up to half of the greenhouse-gas warming. These comparisons suggest that at least 10 times as much sulfate aerosol would be needed in the troposphere as would be needed in the stratosphere for a comparable climatic effect. A better understanding of the role of the already- present aerosols is a prerequisite for further progress in the use of aerosols for climate engineering. The links between the horizontal and vertical distribution of radiative sources and sinks and various atmospheric feedback processes, especially those related to the hydrological cycle and the consequent global and regional responses, are also needed.

1. Introduction

Global greenhouse warming has been developing over the last half century as a major environmental issue, and will become even more serious over the next century, as it continues to increase. Because of the long atmospheric residence time of carbon dioxide and other greenhouse gases, we can only hope to moderately slow down the growth of their atmospheric concentrations over the next century by limiting, or ceasing, the growth of their emission rates (IPCC, 1994, 1995). Only with large reductions in the use of fossil fuels and the other technologies that contribute to the problem would it become possible to stabilize the concentration of these greenhouse gases at levels close to those now present.

There are two conceivable geophysical approaches to reducing or eliminating the radiative impacts of increasing greenhouse gases. If effective, some versions of either might well be of lower economical cost than would be global reductions in fossil fuel use. The first approach would be to promote the removal of the

Climatic Change 33: 279-290, 1996. (~) 1996 Kluwer Academic Publishers. Printed in the Netherlands.

280 ROBERT E. DICKINSON

greenhouse gases from the atmosphere. For example, iron-fertilization might be used to remove substantial amounts of carbon dioxide from the atmosphere (Martin et al., 1994; Monastersky, 1995). Carbon dioxide could also be removed through the growing of trees, but this would only reduce concentrations if the trees were put into permanent storage, or used in place of fossil fuel (e.g., Hall and House, 1994).

The second appproach is to add something further to the atmosphere that would cancel out the radiative effects of greenhouse gases. This could be done by increasing, in some way, the reflection of solar radiation. Suggestions have been made for screening out solar radiation to compensate for warming by: the use of space mirrors, space dust, stratospheric aerosol, or multiple balloon screens; or the use of tropospheric aerosol to change the number of cloud droplets. These suggestions have varying degrees of economic feasibility. The most extensive discussion to date of the various possibilities for climate engineering, including their feasibilities and costs, is given by COSEPUP (1992). This report states that

"Cloud stimulation by provision of cloud condensation nuclei appears to be a feasible and low-cost option capable of being used to mitigate any quantity of CO2 equivalent per year. Details of the cloud physics, verification of the amount of CCN to be added for a particular degree of mitigation, and the possible acid rain or other effects of adding CCN over the oceans need to be investigated.

Several schemes depend on the effect of additional dust (or possibly soot) in the stratosphere or very low stratosphere screening out sunlight. Such dust might be delivered to the stratosphere by various means, including being fired with large rifles or rockets or being lifted by hydrogen or hot-air balloons. These possibilities appear feasible, economical, and capable of mitigating the effect of as much CO2 equivalent per year as we care to pay for."

Because COSEPUP (1992) found that the use of tropospheric or stratospheric aerosols would likely be the cheapest alternatives, further discussion is largely limited to these possibilities. However, many of the points made could generalize to any solar reflector.

As the above quote indicates, tropospheric aerosols not only have direct radiative effects as stratospheric aerosols, but also may act as CCN (cloud condensation nuclei). An important distinction is that aerosols in the troposphere remain in the atmosphere about a week, whereas those in the stratosphere remain at least a year. Thus, the relative ease of delivering materials to the troposphere may be more than canceled out by the significantly larger volume of materials needed. The short atmospheric lifetimes of aerosols compared to greenhouse gases is fortunate, as otherwise we would now live in an aerosol cloud. It implies a much greater reversibility to any future geoengineering schemes using aerosols than we have for greenhouse warming.

CLIMATE ENGINEERING 281

Briefly summarized, on the positive side, actions to screen out solar radiation may be technically and economically feasible, they might be scientifically quantified in terms of their effect on climate and they would be relatively reversible. On the negative side are the likelihood of deleterious environmental consequences and the possibility that serious miscalculations could occur as a result of our lack of understanding of the climate system. Our understanding of the global system is still far from being able to adequately evaluate the consequences of such tinkering.

2. Global Energy Balance Considerations

What happens to solar radiation and how might it be changed? As simplified for easy appreciation by non-technical readers, about one-quarter of the solar radiation incident at the top of the atmosphere is absorbed by the atmosphere, about one-half reaches the earth's surface and about one-quarter is reflected by the atmosphere (as shown in Figure 1). Of the half that reaches the earth's surface, only a small amount is reflected. Therefore, the greatest leverage on changing the reflection of solar radiation is within the atmosphere, through the changing of either cloud or aerosol properties.

Of the solar radiation that reaches the surface, about half is returned to the atmosphere as latent heat, through the evaporation/rainfall cycle. Much of the rest is lost through net thermal radiative cooling to the atmosphere. Upward thermal radiation from the earth's surface alone removes even more energy than is absorbed from the sun, but most of this energy is returned by downward thermal radiation from cloud bases and atmospheric greenhouse gases. If solar radiation is removed from the atmosphere by reflection, there is a corresponding reduction in solar radiation reaching the surface (and surface temperature). Net surface radiation is highly correlated with, and generally the dominant control of, evapotranspiration. Thus, except for water-limited systems, evapotranspiration closely scales with the amount of solar radiation received. It will also increase with a warmer atmosphere, which can hold more water and provides greater downward thermal radiation (which acts to increase surface temperature). Additional control by stomates is most pronounced for forests, where large mechanical mixing weakens somewhat the radiative controls on evapotranspiration.

Zero-dimensional (0-D) climate models are used for simple analyses of how this system might respond to changes in its forcing or in other parameters. The earth atmosphere system absorbs 70% of the incoming solar radiation. The energy is re-emitted as thermal energy by clouds and greenhouse gases at an effective radiation temperature that is 33 ~ colder than that of the surface. It is this 33 ~ difference that determines present surface temperature for the cunently absorbed solar radiation. Its increase by the addition of greenhouse gases acts as would additional heating added to the earth-atmosphere system. In other words, with both the radiating temperature and the 70% kept fixed, the surface temperature will then


What Happens to Incoming Solar Radiation?

114 absorbed by atmosphere

.',-<.. ilk".,/.':'~,, ~ . . . . . . . . . .

" , , " , " ~, 7,;; ....:.i;,.., ::.... .... . ,'. '"'-W'. �9 ...... ~ " ~ ,,,::~...~: 1/4 reflected by

k 7 " .,.

......., , , :::'7 atmosohere

/

?ii:~"! :'':" "...:,..C "i .:,'. ~ ] 2,

1/2 reaches surface

Figure 1. Division of global average solar radiation between atmospheric absorption, reflection, and that reaching the surface.

increase as we 'stretch' the 33 ~ Such changes can be compared with changes in solar input by determining how they change the net radiative flux at the tropopause.


The 0-D climate models assume the vertical tropospheric temperature profile to be a specified function of surface temperature. Convection is presumed to maintain this profile. With radiative fluxes specified at the tropopause, the exchange of energy between the surface and the atmosphere does not need to be considered. With these simplifications, various alternatives for changing global radiation balance can be assessed and compared provided only that the effect they have on radiative fluxes at the tropopause can be calculated. Global energy balance can be imposed as follows: let G be the net of radiative energy fluxes at the top of the atmosphere, G = 0 over a long time average. If the incident global solar flux is Q, albedo is c~, and outward thermal emission is F, then the net absorbed solar flux is

s = Q ( 1 - (1)

and the net radiative flux G is

G = S - F (2)

The assumption that is made to derive a 0-D model is that S and F are functions of global temperature T, either directly or through their dependence on other parameters that in turn depend on T. The strongest justification for this assumption is that it approximately corresponds to the behavior of more detailed models.

Given this assumed dependence on T, a small change in the components of Equation (2) can be written

A G = 6G - A T / ' ) = 0 (3)

where A T is the change in global average temperature, 5G a change in G for reasons other than a change in T, and ')' the sensitivity factor,

') = (4)

Because the 0-D modeling arguments are primarily diagnostic of the results of more elaborate models, they have no independent predictive capability beyond showing in simple terms what the more elaborate models would give for global average temperature. However, they do allow for simple consideration of new suggestions for mechanisms to change global radiative balances. In a somewhat more elaborate form that allows for ocean heat capacity and transport, they have been used by IPCC (1990, 1995) to provide estimates of global average temperature response for various scenarios of future increases of greenhouse gases and tropospheric aerosols. Equation (3) shows how changes of global average temperature can be inferred in terms of the prescribed forcing 6G and the sensitivity factor ')'. The magnitude of different kinds of radiative forcing can be compared in terms of this O-D analysis by their relative contributions to 5G. This is the basis for suggestions that by changing albedo we can cancel out the effects of the climate greenhouse gases.


3. How Much Might Be Needed?

The effect of adding aerosols to the stratosphere can be quantified rather well because of studies of the effects of past volcanoes, which do this same thing naturally. Lacis et al. (1992) have theoretically examined the forcing from volcanic aerosols, and they find "that aerosol forcing of the tropospheric climate is remarkably independent of most aerosol parameters other than optical thickness. An exception is particle size: greenhouse heating by particles larger than 1 #m can rival or exceed the albedo cooling effect". Such large particles, occurring primarily as volcanic ash, drop out of the stratosphere within a few months, and the radius of the remaining particles found from sulfurous volcanic gases is typically about 0.5 #m.

The long-lived volcanic aerosol particles, largely of sulfate, initially depend on how much SO2 gas the volcano can push into the stratosphere. This gas is oxidized within a month or so to sulfuric acid which condenses into very tiny droplets, smaller than 0.1 #m. These droplets grow to larger ones with radii between 0.1 #m and 1 #m; particles in this size range have a maximum effect on solar radiation for a given total mass of aerosol. On a time scale of a year or two, the radiative effects of a volcano begin to decline significantly. Particles are removed by coagulation and transported by large-scale motion and sedimentation to the troposphere. Because coagulation increases non-linearly with aerosol concentrations, larger additions of sulfate aerosol have diminishing radiative effects (e.g., Pinto et al., 1989, find increasing sulfate from 10 to 100 teragrams gives only a four-fold peak radiative effect and a lesser relative increase with later times).

The recent (1991 ) Pinatubo eruption is the most extensively studied of any volcano. Its optical properties have been characterized by surface and satellite remote sensing and its particle sizes and composition determined by direct sampling. Its impact on net radiative fluxes has been measured directly from satellite by Minnis et al. (1993). Zhao et al. (1995) provide a detailed modeling study of the chemical, microphysical and radiative propelxies of the Pinatubo aerosol. Figure 2 from Hansen et al. (1992) shows their preliminary estimate of the time history of the optical depth from Pinatubo and its corresponding climate forcing. The peak optical depth of 0.15 is estimated to correspond to an addition of 20 to 30 teragrams of sulfate aerosols into the stratosphere. Its estimated radiative forcing of almost -4 W/m 2 is approximately the cooling required to cancel the radiative effect of a doubling of CO2.

In the absence of volcanoes, the stratosphere still has a small residual sulfate aerosol. The primary source of this aerosol is the oxidation of carbonyl sulfide (OCS), emitted as a gas from the oceans. The enhancement of these OCS fluxes (e.g., as analyzed by Taubman and Kasting, 1995) might be considered as an alternative to direct delivery of stratospheric aerosol. Only a fraction of this gas reaches the stratosphere. Evidently as much or more sulfur than is supplied to


0.1

0

(a)

1991 1993 1995

a) Global mean aerosol optical depth

AF(WIM 2) 6

5

4

3

2

1

0

(b)

J

99 1993 1995 b) Radiative forcing

Figure 2. Estimate of time history of global mean optical depth and radiative forcing from the Pinatubo volcano (Hansen et al., 1992).

tropospheric aerosols would be required to supply the stratosphere with as much sulfate aerosol as provided by Pinatubo (Taubman and Kasting, 1995).

The annual addition to the troposphere of about 80-100 teragrams of sulfate from fossil-fuel combustion has a near-instantaneous direct effect on radiative forcing of -0 .2 to -0.8 W/m 2 (IPCC, 1995). The mid-value of-0 .5 W/m 2 is about one quarter of that needed to cancel the present greenhouse wzaxning, or about one-eighth of that needed to cancel a doubling of CO2. The global spatial and seasonal variations of tropospheric aerosol, both natural and anthropogenic, are at present poorly known, as are the processes of aerosol formation and removal (Penner et al., 1994; IPCC, 1994, 1995). For a given sulfate mass, climate forcing depends heavily on relative humidity. It also depends on aerosol size distribution and solar zenith angle (Pilinis et al., 1995).

In addition, aerosols may increase cloud brightness. That is, more aerosols acting as CCNs can lead to smaller cloud droplets; for the same amount of water, smaller cloud droplets have more surface area and so reflect more solar radiation. The number of effective CCNs increases less rapidly than does the sulfate aerosol (e.g., Hegg, 1994) so that the magnitude of this effect depends on the details of the spatial distribution of the sulfate aerosols, but overall it is thought to be very approximately the same magnitude as the direct effect. IPCC (1994, 1995) estimates that the indirect radiative forcing from anthropogenic aerosols is between 0 and -1.5 W/m 2, that is very poorly known.


Given the current estimates for the mass budgets and radiative effects of volcanoes, versus annual sulfate emission from fossil fuels and direct and indirect effect of tropospheric sulfate aerosol, it would appear that 10 teragrams per year of sulfate aerosol added directly to the stratosphere would have as much or more climatic effect than 100 teragrams added to the troposphere. Both additions would likely remove -1 to -2 W/m 2 of solar heating; but the 10 teragrams in the stratosphere would probably have an impact close to the upper end of this range and the 100 teragrams in the troposphere near the lower end.

4. Limitations

Fundamental limitations in our present understanding of the climate system discour- age any rapid application of climate engineering through the addition of aerosols. Besides potentially deleterious impacts on ozone concentrations if applied in the stratosphere (e.g. Hofmann and Solomon, 1989; Prather, 1992; Hanson et al., 1994), there have been two general sources of concern:

(1) climate changes over the last century are not well quantified in terms of modeled responses to increasing greenhouse gases, and

(2) the spatial distributions of radiative changes may be important, but depend on a number of poorly understood feedbacks.

4.1. MATCHING OBSERVED WITH MODELED CLIMATE CHANGES

One of the perplexing issues in interpreting the temperature record over the last century has been that the observed temperature increase of 0.5 K is less than half that expected from the conventionally modeled response to the observed increases of greenhouse gases. Some observational clues as to what might be happening in addition to greenhouse warming lie in the not-entirely-smooth behavior of long- term temperature changes over the last century. Much of the total temperature increase occurred before 1940, after which temperatures remained level until the 1960s, and have been rapidly rising since then. This abrupt behavior might seem to suggest the possibility of large natural excursions of climate resulting from system non-linear dynamics. However, very long-time model integrations do not reproduce the large level of variability that has been realized over the last century (Stouffer et al., 1994). Further observational information and analyses suggest an alternative line of reasoning more related to the present subject of this paper.

The rapid release of sulfate aerosols with the large increase in fossil-fuel usage after 1940, and the consequent direct and indirect effects, has alternately been proposed as the explanation for the discrepancies between the best model estimates and the observed record of temperature increase over the last century. This view, simply put, is that the temperature increases prior to 1940 were the response to greenhouse warming, but that the rapid increases in fossil-fuel usage over the

CLIMATE ENGINEERING 2 8 7

next several decades raised the level of sulfate aerosol sufficiently to cancel the radiative effects of greenhouse gases over that period. Eventually, in the 1970s, further growth of fossil-fuel usage slowed to a point where the accumulation of greenhouse gases out-distanced any further build-up of sulfate aerosols, presumably helped along by the push toward low sulfur fuels. If the net radiative effect of tropospheric sulfate aerosols corresponds to current estimates o f - 1 W/m 2, they would be canceling out about half the radiative effects of greenhouse gases. Since this cancellation in forcing applies only during the day, a shift toward relatively warm nights would be expected over continents, where the surface thermal capacity is small enough to allow a strong diurnal cycle.

Andreae (1995) reviews current modeling of the climatic effects of tropospheric sulfate aerosol. Because of the relatively short lifetime of tropospheric aerosol, it is concentrated in the regions of largest fossil fuel use, or downwind from them. Its loading is only approximately known through modeling from the known fossil fuel usage and its sulfur content. Direct measurements from satellite remote sensing at present can indicate aerosol optical depths only for regions of large aerosol concentrations over the oceans. Besides the large uncertainty in atmospheric loading, the radiative effects of a given mass of aerosol are not adequately known, in part because of theft dependence on a poorly-known size distribution.

Partitioning of temperature measurements between day and night, where possible, indicates that the observed temperature increases over continents have been twice as large at night as during the day (IPCC, 1995). Increasing cloud cover provides another, at least a partial, line of explanation for the smallness of the warming over the last century and during the daytime. Trends of increasing cloud cover have been reported by several authors (e.g. Henderson-Sellers, 1986). The intriguing possibility that this increasing cloudiness is tied to increasing tropospheric aerosol, along with indirect effects in general, needs further exploration. The concept of climate engineering through the use of aerosols would be on firmer ground if we had a better understanding of the role of aerosols in determining the temperature record over the last century.

4.2. POSSIBLE IMPORTANCE OF SPATIAL DISTRIBUTIONS OF AEROSOL COOLING

The spatial distribution of radiative changes, which are left out of the 0-D approach, are likely to be important. Poorly-understood feedbacks are involved that include the large number of terms involving different aspects of how the distaibution of water vapor, cloud amount, cloud height, cloud water content, cloud droplet size and cloud geometry will change and affect radiative balances. These in turn may depend on spatial, especially vertical, distribution of heat inputs in addition to their overall global values. Water vapor is the most powerful greenhouse gas, although it is usually not described as such because it is part of the natural system and not added by humans.


Two recent GCM simulations have reported contradictory concusions as to the importance of the spatial distribution of heating for global temperature response. Taylor and Penner (1994) report a large difference in modeled global sensitivities to aerosol versus greenhouse-gas forcing, whereas preliminary simulations of time evolving greenhouse warming including aerosols by Cox et al. (1995) report that "changes of the global annual mean surface air temperature depend only on global average net forcing and are not highly sensitive to either the details of the seasonal and spatial patterns in forcing or to the nature of the forcing."

IPCC (1995) indicates very different spatial patterns of climate response when the aerosols are included, compared to when they are not. In particular, the patterns with aerosol correspond more closely to the observed climate-warming patterns. Some important parameters even change sign. For example, substantial amplifica- tion of the southeastern Asian summer monsoon is changed to a weakening (IPCC, 1995; Mudur, 1995). In the presence of clouds, there is substantial variation in the vertical and horizontal distributions of the heating or cooling not only from the tropospheric aerosols but also from stratospheric aerosol and greenhouse gases. These inhomogeneities increase the likelihood that cancellation of individual changes in global average fluxes at the top of the atmosphere may not eliminate significant climate changes resulting from lack of local cancellations.

The indirect cloud-brightening effects of aerosol are especially dependent on details of aerosol distributions, presumably one reason that IPCC (1995) appears to be less sanguine about estimating the magnitude of this effect than COSEPUP (1992). Simply put, cloud brightening may be large for a given aerosol in pristine oceanic environments but small in already heavily polluted continental areas. Thus any serious consideration of this technology needs to include the most effective locations for its application.

5. Conclusions

On a technical and economic basis, climate engineering may be feasible. However, the current state of science is not capable of assessing all of the potential side effects. Over the next decade, research on global climate change should build the framework necessary for such an assessment. We need a fuller understanding of the temporal and spatial characteristics of the observed global warming and we need to better quantify how natural and anthropogenic aerosols form and affect the climate system. We need a clearer understanding of the implications of the spatial distributions of heating that would be imposed, how they might effect surface- atmosphere coupling, and the hydrological cycle. We also need a clear assessment of the potential negative effects on the ozone layer and strategies to minimize these.

All of the arguments used here to discuss the climate system have implicitly assumed a simple, linear system. It is pushed in a certain way and an, almost, linear response is anticipated. If nonlinear thresholds in the system could be reached


by advertent or inadvertent climate modification, some very different lines of reasoning would be required.

Acknowledgment

Funding for this work has been received from the NASA EOS Interdisciplinary Sci- entific Research Program (U.P.N. 429-81-22: U.P.N. 428-81-22), through NCAR Interagency Agreement number W- 17, 661.

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(Received 17 April 1995; in revised form 31 January 1996)

climate engineering a review of aerosol approaches to changing the global energy balance

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